Most offshore oil and gas production is conducted from platforms secured to the ocean bottom. A key design constraint for such platforms is that there be no substantial dynamic amplification of the platform's response to waves. This is accomplished by designing the platform to have natural vibrational periods which do not fall within that portion of the range of wave periods representing waves of significant energy. The several modes of platform vibration which are generally of greatest concern in platform design are pivotinf of the structure about the base (commonly termed "sway"), flexure ("bending") in the vertical plane, and torsion about the vertical axis. For most offshore locations the range of natural vibrational periods to be avoided is from 7 to 25 seconds, this representing the range of wave periods occurring with the greatest frequency.
For water depths up to about 300 meters, the technology for avoiding dynamic amplification of an offshore structure's wave response is quite well developed. Nearly all existing offshore structures designed for use in such water depths are fixedly secured to the ocean bottom and stiffened to cause each of the natural vibrational periods to be less than about 7 seconds. Such offshore structures are referred to as "rigid structures". However, as water depths exceed 300 meters, the tonnage of structural steel required to maintain sufficient platform rigidity to ensure that all natural vibrational periods remains below 7 seconds increases rapidly with depth. It has been suggested that for even the richest offshore oil fields the use of a rigid structure could not be economically justified in water depths exceeding about 420 meters due to the limitations imposed by the natural vibrational periods.
For deepwater applications, it has been proposed to depart from conventional rigid structure design and develop platforms having a sway period greater than the range of periods of ocean waves containing significant energy. Consequently, much of the environmental load imposed on the platform is resisted by its own inertia. Such platforms are termed "compliant structures." The use of a compliant platform effectively removes the upper bound on the sway period. This greatly reduces the increase in the structural steel, and hence cost, required for a given increase in water depth.
In one type of compliant structure, the guyed tower, the platform deck is supported on a slender space-frame structure extending from the ocean bottom to the ocean surface. A radially arranged set of guylines extend outward from an upper portion of the space-frame structure to the ocean bottom. These guylines provide a restoring force to counter platform sway induced by environmental forces. Guyed towers are disadvantageous in that the guyline system is expensive to fabricate and deploy. In certain applications the guylines may also present an obstacle to navigation and fishing in the vicinity of the platform.
A second type of compliant structure, the tension leg platform, uses buoyancy to provide a restoring force to resist the platform's lateral displacement. The deck of the tension leg platform is situated on a large buoyant hull which is secured to a foundation at the ocean bottom by a set of vertical tethers. The tethers are tensioned and hence maintain the hull at a deeper draft than it would assume if floating free. When the hull is displaced laterally by environmental forces, the net vertical buoyant force acting on the tethers produces a righting moment tending to restore the hull to its original vertical position.
A significant drawback of the tension leg platform is that its buoyancy requirements are great. This necessitates use of a large and expensive hull structure. This is undesirable in that it increases the cross sectional area of the structure exposed to wind, waves, and current. Additionally, the production wells system for a tension leg platform is substantially more complex than that required for a traditional rigid structure. Further, for use in water depths greater than about 600 meters it is highly desirable to provide the tethers with inherent buoyancy to minimize the loading the tethers impose on the hull. This presents numerous technical problems.
It would be desirable to develop a compliant tower which does not rely primarily on guylines or positive buoyancy to counter lateral displacement caused by environmental forces.